1. Field of the Invention
The present invention relates to copper base alloys, and to methods for producing a casting and a forging employing these copper base alloys.
This application is based on Japanese Patent Application No. 2000-103662, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Metallic materials that are high strength and high thermal conductivity are employed in fields where materials are subjected to severe thermal fatigue, such as in the case of structural materials of forming nuclear fusion reactors and thrust chambers of a rocket engine where one surface is in contact with 3000xc2x0 C. combustion gas and the other surface is in contact with liquid hydrogen.
A copper base alloy containing 0.8% Cr and 0.2% Zr (note that xe2x80x9c%xe2x80x9d as employed in this specification hereinafter indicates mass %) that is disclosed in Japanese unexamined Patent Application, First Publication No. Hei 04-198460 may be cited as an example of a high strength high thermal conductivity alloy that is employed in these fields. In general, a high strength, high thermal conductivity forging can be obtained from this copper base alloy by casting it, and then forming it into a specific shape by forging, rolling, etc. while applying a specific heat treatment thereto. In this copper base alloy, it is possible to increase the tensile strength while maintaining thermal conductivity at a high level, even if the alloy""s composition is the same, by adjusting the conditions of the thermomechanical treatment.
In recent years, however, the conditions under which structural components are employed have become severe with respect to the generation of thermal stress. At the same time, the short lifespan of conventional materials before cracking occurs has been pointed out. Thus, there has been a demand for higher resistance to thermal fatigue. To reduce thermal strain in metallic materials, it is necessary to increase thermal conductivity as well as to improve thermal fatigue strength. Improvements in thermal conductivity are near the limits, however. Thus, the challenge is to improve thermal fatigue strength without reducing thermal conductivity as compared to conventional metal materials.
It is known that in order to increase thermal fatigue strength in these types of metal materials, it is generally acceptable to increase tensile strength and tensile proof stress without reducing tensile elongation and thermal conductivity at the employed temperatures. Therefore, in order to meet the aforementioned demands, an attempt was made to increase strength by employing a copper base alloy that contained Cr (0.8%) and Zr (0.2%) as the base, and then increasing the draught of the copper base alloy by further increasing the Cr and Zr contents.
In this type of Cuxe2x80x94Crxe2x80x94Zr alloy, a high degree of strength can be obtained if the Cr and Zr contents are increased, while at the same time generating a fiber-type microstructure by swaging or wire drawing, which apply a large deformation in one direction.
Ductility is reduced in this type of Cuxe2x80x94Crxe2x80x94Zr alloy, however, so that thermal fatigue strength did not improve as much as expected. Moreover, there are limitations on the shape of the formed article, so that a sufficient amount of forging and rolling cannot be carried out. Thus, it was difficult to obtain the desired strength for a formed article of an optional shape. Accordingly, applications were limited to electrical parts utilizing high strength and electroconductivity.
On the other hand, a Cuxe2x80x94Ag alloy in which a large amount of Ag is added has been developed as a new alloy, as disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 6-279894 and in SAKAI, et al: J. JAPAN INST. METALS, Vol. 55, No. 12 (1991), pp. 1382-1391. Ag, like Cr and Zr, has little solubility in Cu near room temperatures, and experiences little reduction in thermal conductivity when rendered into an alloy. However, if Ag is added in an amount of 8.5% or more, then the obtained copper base alloy forms eutectic when solidifying. Thus, if swaging or wire drawing, which apply a large deformation in one direction, are carried out in the same manner as in the case of a Cuxe2x80x94Crxe2x80x94Zr alloy to an ingot of a Cuxe2x80x94Ag alloy in which 15% Ag has been added in order to obtain a sufficient amount of eutectic structure, the eutectic structure is destroyed and a fiber reinforced structure is generated. The strength obtained in this case is extremely high.
However, in the case of this type of Cuxe2x80x94Ag alloys, sever working such as to obtain a wire rod with a {fraction (1/10)} or smaller diameter from a forged round bar is required. Thus, it is not possible to produce wrought articles of greater than a certain degree of thickness with this technology.
In addition, in the above-described metal materials, repetition of forging and heating treatments increase production costs. Accordingly, since strength on par with current levels is sufficient, there has been a desire for a metal material that is high thermal conductivity, high strength, and inexpensive that can be produced by means of casting where a forging step is not required. However, this type of metal material has not been conventionally known.
The present invention was conceived in view of the above-described problems, and has as it objective the provision of a metal material that enables the inexpensive production of a high strength, high thermal conductivity metal formed article by means of simple casting, forging or rolling, in which there are no limitations on the dimensions of the formed article""s shape. It is the further objective of the present invention to provide a method of production for a metal formed article employing this metal material.
The present invention provides a copper base alloy (also called xe2x80x9ccopper base alloy for castingxe2x80x9d), that contains Ag in the range of 3 to 20%, Cr in the range of 0.5 to 1.5%, and Zr in the range of 0.05 to 0.5%, with Cu comprising the remainder.
The present invention also provides a method for producing a casting that includes a first step for melting a copper base alloy containing Ag in the range of 3 to 20%, Cr in the range of 0.5 to 1.5%, and Zr in the range of 0.05 to 0.5%, with Cu comprising the remainder; a second step for casting the molten material obtained in the first step into a specific shape by rapidly solidifying; and a third step for precipitation strengthening the formed article obtained in the second step by carrying out aging treatment at a temperature in the range of 450 to 500xc2x0 C.
The phrase xe2x80x9crapidly solidifyingxe2x80x9d as used here means that the time required to cool the temperature of the molten material to 450 to 500xc2x0 C., which is the temperature of the aging treatment for precipitation, is 10 minutes or less. Alternatively, this phrase means solidifying using a metal mold that can cool the material to 500xc2x0 C. at a rate of roughly 1xc2x0 C./sec once the material is solidified. Specifically, metal-mold casting methods or centrifugal casting methods are available for this purpose.
The phrase xe2x80x9caging treatment for precipitationxe2x80x9d means a treatment to precipitate different phases within a matrix by holding a solid solution at a specific temperature for a specific duration of time.
A copper base alloy for casting of the aforementioned components is formed by adding Ag to a copper base alloy in which a small amount of Cr and Zr have been added. This copper base alloy makes it possible to obtain a formed article that is high strength and thermal conductivity, even in the case of casting where rolling and forging are not required.
Accordingly, if this copper base alloy for casting is employed, a casting that is high strength and thermal conductivity in which there are no limitations on the dimensions of the article""s shape can be produced through the simple operation of casting.
When the amount of Ag is less than 3% in a copper base alloy of these components, then there is a marked reduction in the hardness of the casting obtained, and a high strength, high thermal conductivity casting cannot be achieved. On the other hand, there is no marked difference in effects when the amount of Ag employed exceeds 20%, and use of excessive amounts of Ag is disadvantageous from the perspective of cost.
When the amount of Cr is less than 0.5% in the copper base alloy of the above components, then there is a marked reduction in the hardness of the casting obtained, and it is not possible to achieve a high strength, high thermal conductivity casting. The maximum solubility of Cr is 0.7 to 0.8%. The eutectic reaction will occur if Cr is added in excess of this range. However, even at amounts exceeding this range, for example, in an alloy in which 1.5% Cr has been added, solidification is complete before the entire eutectic reaction has occurred provided that the cooling speed is not very slow. However, when the amount of Cr exceeds 1.5%, then an excessive amount of Cr primary crystals precipitate out during cooling at the second step. This is not desirable from the perspective of toughness and ductility.
When the amount of Zr in a copper base alloy of the above components is less than 0.05%, then the effect of reducing embrittlement at 400 to 600xc2x0 C. is not sufficient. Moreover, like Cr, Zr is an effective element with respect to precipitation strengthening. The maximum solubility is 0.15%. Adding a large amount of Zr in excess of 0.5% is disadvantageous for the same reasons as cited above in the case of Cr.
In the aforementioned method for producing a casting, a supersaturated solid solution containing a forced solid solution of Ag and Cr is first formed by rapidly solidifying molten material by centrifugal casting or metal-mold casting in the second step. A structure containing a supersaturated solution of Ag in excess of its solubility can be obtained by rapidly solidifying at this stage, even when Ag is added in an amount exceeding 8.5%, which is the point of Agxe2x80x94Cu eutectic formation in the phase diagram. This contributes to strengthening.
The obtained casting contains a considerable amount of Ag forced in solution. As a result, when an aging treatment for precipitation is carried out in the third step, a large amount of fine precipitates are precipitated during aging, thereby increasing the degree of strength of the casting.
The present invention also provides a copper base alloy (also referred to as a xe2x80x9ccopper base alloy for forgingxe2x80x9d to distinguish from the aforementioned xe2x80x9ccopper base alloy for castingxe2x80x9d) that includes Ag in the range of 3 to 8.5%, Cr in the range of 0.5 to 1.5% and Zr in the range of 0.05 to 0.5%, with Cu comprising the remainder.
The present invention further provides a method for producing a forging that includes a first step for melting a copper base alloy for forging; a second step for solidifying the molten material obtained in the first step by casting; and a third step for deforming the solidified article or the hot worked article thereof that was obtained in the second step into a specific shape and also precipitation strengthening, which is obtained by thermomechanical treatment using forging or rolling and aging treatment for precipitation.
The aforementioned copper base alloy for forging has the above composition. As a result, a wrought article is obtained that has superior strength and thermal conductivity, which can be formed through a simple operation and is not limited with respect to the dimensions of its shape, while at the same time employing inexpensive Cu as the base.
When the amount of Ag is less than 3% in the aforementioned copper base alloy for forging, the hardness of the obtained forging decreases markedly, and a high-strength, high-thermal conductivity forging cannot be obtained. On the other hand, there is only a slight effect obtained from adding Ag in amounts in excess of 8.5%, while this approach is disadvantageous from a cost perspective.
When the amount of Cr is less than 0.5% in the copper base alloy for forging, then the hardness of the obtained forging decreases markedly and it is not possible to obtain a high-strength, high-thermal conductivity forging. When the amount of Cr exceeds 1.5%, then a large primary crystal of Cr is generated in the second step and forgeability during hot forging falls off markedly.
When the amount of Zr is less than 0.05% in the Cu alloy for forging, there is insufficient control over embrittlement. On the other hand, when the amount of Zr exceeds 0.5%, then, as in the case of Cr, toughness and ductility decrease due to excessive precipitation.
By carrying out a thermomechanical treatment using forging or rolling to the solidified article obtained in the second step in this method for producing a forging, the crystal grains are made finer, dislocation is introduced and hardening occurs. By also employing an aging treatment for precipitation at the same time, a fine eutectic phase is uniformly generated, making it possible to further increase the strength of the forging. Thus, a high strength, high thermal conductivity forging can be obtained.
In the third step, it is preferable to carry out the thermomechanical treatment at a warm or cold of 550xc2x0 C. or less. When the temperature exceeds 550xc2x0 C., not only is there little work hardening, but the Ag or Cr precipitates are partially dissolved, so that larger precipitates occur, which is inconvenient. Once formed, large precipitates do not readily become finer, even if the temperature is reduced. Thus, precipitation strengthening is markedly decreased.
Next, an explanation in greater detail will be made of the requirements for achieving a high degree of strength and conductivity in the casting obtained from the present invention""s Cu alloy for casting and the forging obtained from the present invention""s copper base alloy for forging.
When producing a casting employing the present invention""s copper base alloy, the molten material consisting of a copper base alloy containing Ag is rapidly solidified by centrifugal casting or mold casting. As a result, a supersaturated solid solution containing a forced solid solution of Ag and Cr is first generated. An aging treatment for precipitation is then carried out to this supersaturated solid solution at a temperature in the range of 450 to 500xc2x0 C. As a result, very fine phases in the solid solution structure are precipitated. The amount of supersaturation in the copper base alloy becomes considerable due to rapidly solidifying. Thus, the amount of fine precipitates that are formed during aging increases, so that the strength of the casting increases.
Unlike in the usual phase diagram showing the structure in the equilibrium phase, in a rapidly solidified copper base alloy, a structure is obtained that contains a higher than anticipated solid solution of Ag. Accordingly, the amount of Ag added is effectively employed in strengthening, even when added in excess of 8.5%, which is the point of eutectic formation in the phase diagram. However, when Ag is added in excess of 20%, the solidifying speed necessary for strengthening is too large. Thus, this is not realistic, and reduces the actual efficacy.
On the other hand, in the present invention""s method for producing a forging, the aforementioned copper base alloy for forging is formed to the desired shape by thermomechanical treatment using forging or rolling, and is subjected to precipitation strengthening using an aging treatment for precipitation. In this method, the amount of Ag added must be adjusted so that many Ag eutectic or Cr primary crystals are not generated. In other words, a structure in which large eutectic or primary crystal Cr appears during initial casting and solidifying because a large amount of Ag was added will cause a reduction in the efficiency of forging during hot forging. For example, in an alloys comprising just the two elements of Cu and Ag, melting begins at a eutectic temperature of 780xc2x0 C. based on the typical phase diagram. This partial melting is the cause of cracking during hot working in the forging or rolling steps. Thus, it becomes necessary to place a restriction on the upper limit of the forging temperature.
Therefore, in order to prevent an excessive amount of large eutectic particles or primary Cr particles from being formed during casting and solidifying in the second step, the amount of Ag added is restricted to less than 8.5%, which is the point of eutectic formation in the phase diagram, in the present invention""s copper base alloy for forging. As a result, the efficiency for forging the present invention""s forging is greatly improved.
In a specific example of the present invention""s method for producing a forging, strength is increased through precipitation strengthening by a thermomechanical treatment with warm working (i.e., a temperature in excess of 100xc2x0 C. and less than 550xc2x0 C., and preferably below 500xc2x0 C.), or cold working (room temperature to 100xc2x0 C.) and aging treatment. In order to increase strength by precipitation strengthening, the particle diameter of the precipitate in the structure is ideally on the order of 1/100 xcexcm. However, by limiting the amount of Ag added to be 8.5% or less, and carrying out the thermomechanical treatment and the aging treatment for precipitation during warm or cold working, a high strength forging can be obtained in which different phase particles of the desired diameter are dispersed.
The two strengthening mechanisms of adjusting the amount of Ag and Cr added and the thermomechanical treatment are mutually promoting. In other words, the dislocation introduced in the thermomechanical treatment becomes the nucleation site for precipitating different phase particles, and contributes to precipitation of fine particles. In addition, the Ag or Cr precipitate in the dislocation limits the elimination of the dislocation by heating, thereby increasing the high temperature strength stability. The more alloy elements, the larger the effect. However, many of these elements precipitate out as primary crystals during casting/solidifying either alone or in a compound phase. Thus, employment of large amounts of these elements leads to a deterioration in forgeability in the later steps. For example, in a Cuxe2x80x94Cr two element alloys, when the amount of Cr added exceeds approximately 0.7%, a primary crystal precipitates out in the case of solidification maintaining the equilibrium phase. Accordingly, the suitable amount of Cr added is 0.7% or less at the equilibrium phase. However, since the speed of solidification is rapid in actuality, it is possible to increase the degree of strength by adding up to 1.5%.
By adding a suitable amount of Cr to the present invention""s copper base alloy for forging, the same effects are obtained as when a large amount of Ag is added. Thus, forging efficiency can be increased and the amount of Ag added can be decreased, so that costs are reduced.
When adjusting the copper base alloy for casting or forging, Ag, Cr and Zr are added to Cu, and melted using the usual method. By adding a suitable amount of Cr, in the range of 0.5 to 1.5%, rather than adding Ag alone, the effect of adding Ag is synergistically increased. Adding Cr in an amount less than 0.5% has only a small effect on improving strength.
Regarding the addition of Zr to a copper base alloy, it has been conventionally known that addition of 0.05 to 0.2% Zr has a deoxidizing effect and the effect of controlling the shape of the grain boundary precipitate. However, the addition of 0.05 to 0.5% Zr in the present invention also contributes to an improvement in tensile ductility at 400xc2x0 C. and higher.